Candida albicans is the most common etiologic agent of oropharyngeal candidiasis (OPC), an opportunistic infection in HIV v patients occurring in as many as 90% of AIDS patients at some point during the course of HIV disease. OPC is also seen at an appreciable rate in head and neck cancer patients due to impaired salivary function resulting from radiation therapy. A salient characteristic of C. albicans is the ability to grow in different morphological forms, from ovoid yeasts to filamentous hyphae. The expression of several virulence factors is intimately associated with morphology, and strains impaired in morphological transitions have reduced virulence in animal models. The molecular mechanism of how C. albicans coordinates expression of virulence genes with morphological forms is unclear. The recently described phenomenon of gene silencing termed post-transcriptional gene silencing (PTGS) triggered by double-stranded RNA (dsRNA) appears to be a conserved mechanism involved in resistance to viruses, protection of the genome from repetitive elements, developmental timing, and heterochromatin silencing. The PTGS effect is achieved by the degradation of mRNA termed RNA interference (RNAi). Genetic studies across many species have identified several homologous genes involved in RNAi, and in every organism where a PTGS function has been described, a member of the Argonaute protein family has been found. The finding of an ARGONAUTE gene homolog in C. albicans suggests that the fungus is capable of PTGS. This novel gene regulatory system may prove to be important in C. albicans for the regulation of virulence genes indirectly by affecting morphological transitions. The goals of this exploratory/developmental proposal are: (1) to uncover evidence in C. albicans of gene regulation by PTGS through RNAi; and (2) to determine the role of PTGS in control of virulence attributes through morphological transitions, and in maintenance of centromeric heterochromatic structure. The proposed initial studies will take genetic and molecular approaches to determine the capability of C. albicans to perform PTGS through analyses of RNAi constructs and gene knock out strains. Although the role of RNAi in the biology of C. albicans is unknown, the probability is high that novel gene regulatory mechanisms will be found that are common to other fungal pathogens.